Image readout lens and image readout apparatus using the same

ABSTRACT

An imaging lens comprises, successively from an object side, a positive first lens L 1  having a convex surface directed onto the object side, a negative second lens L 2  having a concave surface directed onto an image side, a third lens L 3  of a positive meniscus form having a convex surface directed onto the object side, a fourth lens L 4  of a positive meniscus form having a convex surface directed onto the image side, a negative fifth lens L 5  having a concave surface directed onto the object side, and a positive sixth lens L 6  having a convex surface directed onto the image side, the first lens L 1  and the second lens L 2  being cemented to each other, the imaging lens satisfying the following conditional expressions: 
     
       
         0.51&lt; f   34   /f&lt;0.72   
       
     
     
       
         0.83&lt; f   1   /f   6 &lt;1.17 
       
     
     
       
         1.17&lt;( v   1   ×v   3 ) ½   /v   2 &lt;1.62 
       
     
     where 
     f is the focal length of the whole system; 
     f 34  is the composite focal length of the third and fourth lenses; 
     f 1  and f 6  are the respective focal lengths of the first and sixth lenses; and 
     v 1 , v 2 , and v 3  are the respective Abbe numbers of materials of the first to third lenses.

RELATED APPLICATIONS

This application claims the priority of Japanese Patent Application No. 11-056567 filed on Mar. 4, 1999, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image readout apparatus such as facsimile machine, image scanner, or the like; and an imaging lens for image readout mounted with an optical system thereof. In particular, the present invention relates to a five-group, six-element imaging lens in which first and second lenses are cemented to each other.

2. Description of the Prior Art

Conventionally, a solid-state image sensor made of CCD has been used in general as a photosensitive member disposed in an imaging section of an image readout apparatus such as facsimile machine, scanner, or the like. Recently, as the pixel density in CCD has rapidly been increasing, imaging lenses with a higher resolution have been demanded as those employed in the image readout apparatus.

As an imaging lens which can respond to such a demand, a six-element image readout apparatus disclosed in Japanese Unexamined Patent Publication No. 10-253881 has been known.

In the prior art disclosed in the above-mentioned publication, however, a greater depth is hard to obtain as the resolution of the imaging lens becomes higher, whereby a higher degree of focus adjustment is required in the image readout apparatus.

Also, there has been a fear of the resolution deteriorating due to fluctuations in flatness of an original or due to the flotation thereof.

SUMMARY OF THE INVENTION

In view of such circumstances, it is an object of the present invention to provide a six-element imaging lens which can reduce curvature of field and enhance the depth, thereby alleviating the burden of focus adjustment in an image readout apparatus, and can lower the deterioration in resolution due to fluctuations in flatness of the original or due to the flotation thereof.

It is another object of the present invention to provide an image readout apparatus using such an imaging lens.

In one aspect, the imaging lens in accordance with the present invention comprises, successively from an object side, a positive first lens having a convex surface directed onto the object side, a negative second lens having a concave surface directed onto an image side, a third lens made of a positive meniscus lens having a convex surface directed onto the object side, a fourth lens made of a positive meniscus lens having a convex surface directed onto the image side, a negative fifth lens having a concave surface directed onto the object side, and a positive sixth lens having a convex surface directed onto the image side, the first and second lenses being cemented to each other, the imaging lens satisfying the following conditional expressions (1) to (3):

0.51<f ₃₄ /f<0.72  (1)

0.83<f ₁ /f ₆<1.17  (2)

1.17<(v ₁ ×v ₃)^(½) /v ₂<1.62  (3)

where

f is the focal length of the whole system;

f₃₄ is the composite focal length of the third and fourth lenses;

f₁ is the focal length of the first lens;

f₆ is the focal length of the sixth lens;

v₁ is the Abbe number of a material of the first lens;

v₂ is the Abbe number of a material of the second lens; and

v₃ is the Abbe number of a material of the third lens.

In another aspect, the imaging lens in accordance with the present invention comprises, successively from an object side, a positive first lens having a convex surface directed onto the object side, a negative second lens having a concave surface directed onto an image side, a third lens made of a positive meniscus lens having a convex surface directed onto the object side, a fourth lens made of a positive meniscus lens having a convex surface directed onto the image side, a negative fifth lens having a concave surface directed onto the object side, and a positive sixth lens having a convex surface directed onto the image side, the first and second lenses being cemented to each other, the second and fifth lenses satisfying the following conditional expression (4):

θ_(g,F)+0.0019v _(d)<0.650  (4)

where

θ_(g,F) is a partial dispersion ratio of a lens material expressed by:

θ_(g,F)=(N _(g) −N _(F))/(N _(F) −N _(C))

v_(d) is an Abbe number of the lens material expressed by:

v _(d)=(N _(d)−1)/(N _(F) −N _(C))

where

N_(g) is the refractive index of the lens material at a wavelength of 435.8 nm;

N_(F) is the refractive index of the lens material at a wavelength of 486.1 nm;

N_(C) is the refractive index of the lens material at a wavelength of 656.3 nm; and

N_(d) is the refractive index of the lens material at a wavelength of 587.6 nm.

In still another aspect, the imaging lens in accordance with the present invention comprises, successively from an object side, a positive first lens having a convex surface directed onto the object side, a negative second lens having a concave surface directed onto an image side, a third lens made of a positive meniscus lens having a convex surface directed onto the object side, a fourth lens made of a positive meniscus lens having a convex surface directed onto the image side, a negative fifth lens having a concave surface directed onto the object side, and a positive sixth lens having a convex surface directed onto the image side, the first and second lenses being cemented to each other, the first and sixth lenses satisfying the following conditional expression (5):

N _(d)+0.015v _(d)>2.58  (5)

where

N_(d) is the refractive index of a lens material at d-line; and

v_(d) is the Abbe number of the lens material.

The imaging lens in accordance with the present invention satisfying the above-mentioned conditional expressions (1) to (3) may be configured so as to satisfy at least one of the above-mentioned conditional expression (4) concerning the second and fifth lenses and the above-mentioned conditional expression (5) concerning the first and sixth lenses.

Also, the above-mentioned imaging lens may be configured so as to satisfy the following conditional expression (6):

φ₆≦φ₅≦φ₄≦φ₃≦φ₁₂ or φ₁₂≦φ₃≦φ₄≦φ₅≦φ₆  (6)

where

φ₁₂ is the larger outside diameter in any of the first and second lenses;

φ₃ is the outside diameter of the third lens;

φ₄ is the outside diameter of the fourth lens;

φ₅ is the outside diameter of the fifth lens; and

φ₆ is the outside diameter of the sixth lens.

Further, the above-mentioned imaging lens may be configured so as to satisfy the following conditional expression (7):

0.05≦|β|≦0.7  (7)

where

β is the magnification of the whole system.

The image readout apparatus of the present invention is characterized in that it uses any of the above-mentioned imaging lenses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a basic lens configuration in accordance with Examples 1 to 6 of the present invention;

FIGS. 2A, 2B, 2C and 2D are aberration charts (showing spherical aberration, astigmatism, distortion, and chromatic aberration in magnification) of the lens in accordance with Example 1;

FIGS. 3A, 3B and 3C are aberration charts (showing lateral aberration) of the lens in accordance with Example 1;

FIGS. 4A, 4B, 4C and 4D are aberration charts (showing spherical aberration, astigmatism, distortion, and chromatic aberration in magnification) of the lens in accordance with Example 2;

FIGS. 5A, 5B and 5C are aberration charts (showing lateral aberration) of the lens in accordance with Example 2;

FIGS. 6A, 6B, 6C and 6D are aberration charts (showing spherical aberration, astigmatism, distortion, and chromatic aberration in magnification) of the lens in accordance with Example 3;

FIGS. 7A, 7B and 7C are aberration charts (showing lateral aberration) of the lens in accordance with Example 3;

FIGS. 8A, 8B, 8C and 8D are aberration charts (showing spherical aberration, astigmatism, distortion, and chromatic aberration in magnification) of the lens in accordance with Example 4;

FIGS. 9A, 9B and 9C are aberration charts (showing lateral aberration) of the lens in accordance with Example 4;

FIGS. 10A, 10B, 10C and 10D are aberration charts (showing spherical aberration, astigmatism, distortion, and chromatic aberration in magnification) of the lens in accordance with Example 5;

FIGS. 11A, 11B and 11C are aberration charts (showing lateral aberration) of the lens in accordance with Example 5;

FIGS. 12A, 12B, 12C and 12D are aberration charts (showing spherical aberration, astigmatism, distortion, and chromatic aberration in magnification) of the lens in accordance with Example 6;

FIGS. 13A, 13B and 13C are aberration charts (showing lateral aberration) of the lens in accordance with Example 6; and

FIG. 14 is a schematic view showing an image readout apparatus in accordance with an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, an embodiment of the imaging lens in accordance with the present invention will be explained with reference to specific examples.

In this embodiment, as shown in FIG. 14, the imaging lens acts as an image readout lens 10 used in an optical system of an image readout apparatus 20 such as image scanner. In this image readout apparatus 20, the image readout lens 10 is disposed between a glass plate 4 for mounting an original 3 and a CCD cover glass 6 of a linear CCD 5 in which CCDs are arranged in one to several rows, whereas an illuminating device 7 is disposed on the image readout lens 10 side of the glass plate 4.

As the original 3 is moved in directions A perpendicular to the aligning direction of the linear CCD relative to the optical axis of the imaging lens, images on the original 3 are read out.

FIG. 1 shows the basic lens configuration of Examples 1 to 6. As shown in FIG. 1, the imaging lens for image readout (image readout lens) in accordance with these examples is a five-group, six-element lens system of ortho-meta type constituted by six lenses L₁ to L₆, in which the first lens L₁ and the second lens L₂ are cemented to each other, whereas a stop 2 is disposed between the third lens L₃ and the fourth lens L₄, whereby a luminous flux incident along the optical axis X from the object side forms an image at an imaging position P on an imaging surface 1.

Here, the first lens L₁ is a positive meniscus lens having a convex surface directed onto the object side, the second lens L₂ is a negative meniscus lens having a concave surface directed onto the image side, the third lens L₃ is a positive meniscus lens having a convex surface directed onto the object side, the fourth lens L₄ is a positive meniscus lens having a convex surface directed onto the image side, the fifth lens L₅ is a negative meniscus lens having a concave surface directed onto the object side, and the sixth lens L₆ is a positive meniscus lens having a convex surface directed onto the image side.

Also, the image readout lens in accordance with each Example satisfies the following conditional expressions:

0.51<f ₃₄ /f<0.72  (1)

0.83<f ₁ /f ₆<1.17  (2)

1.17<(v ₁ ×v ₃)^(½) /v ₂<1.62  (3)

where

f is the focal length of the whole system,

f₃₄ is the composite focal length of the third and fourth lenses;

f₁ is the focal length of the first lens,

f₆ is the focal length of the sixth lens,

v₁ is the Abbe number of the glass material of the first lens,

v₂ is the Abbe number of the glass material of the second lens, and

v₃ is the Abbe number of the glass material of the third lens;

θ_(g,F)+0.0019v _(d)<0.650  (4)

where

θ_(g,F) is a partial dispersion ratio of a glass material in the second and fifth lenses expressed by:

θ_(g,F)=(N _(g) −N _(C))/(N _(F) −N _(C))

v_(d) is an Abbe number of the glass material in the second and fifth lenses expressed by:

v _(d)=(N _(d)−1)/(N _(F) −N _(C))

where

N_(g) is the refractive index of the lens glass material at a wavelength of 435.8 nm,

N_(F) is the refractive index of the lens glass material at a wavelength of 486.1 nm,

N_(C) is the refractive index of the lens glass material at a wavelength of 656.3 nm, and

N_(d) is the refractive index of the lens glass material at a wavelength of 587.6 nm;

N _(d)+0.015v _(d)>2.58  (5)

where

N_(d) is the refractive index of a glass material in the first and sixth lenses at d-line, and

v_(d) is the Abbe number of the glass material in the first and sixth lenses;

φ₆≦φ₅≦φ₄≦φ₃≦φ₁₂ or φ₁₂≦φ₃≦φ₄≦φ₅≦φ₆  (6)

where

φ₁₂ is the larger outside diameter in any of the first and second lenses,

φ₃ is the outside diameter of the third lens,

φ₄ is the outside diameter of the fourth lens,

φ₅ is the outside diameter of the fifth lens, and

φ₆ is the outside diameter of the sixth lens; and

0.05≦|β|≦0.7  (7)

where

βis the magnification of the whole system.

Technical significance of each of the above-mentioned conditional expressions (1) to (7) will now be explained.

The above-mentioned conditional expression (1) is one defining the ratio f₃₄/f of the composite focal length f₃₄ of the third lens L₃ and fourth lens L₄ to the focal length f of the whole system, in order to favorably correct spherical aberration, curvature of field, and astigmatism.

In this conditional expression (1), if the value of f₃₄/f is less than the lower limit, then curvature of field becomes greater, and spherical aberration is corrected in excess, whereby image quality deteriorates. If the value of f₃₄/f exceeds the upper limit, by contrast, then astigmatism becomes greater, and spherical aberration is corrected insufficiently.

The above-mentioned conditional expression (2) is one defining the ratio f₁/f₆ of the focal length of the first lens L₁ to that of the sixth lens L₆, in order to favorably correct distortion and chromatic aberration in magnification.

In this conditional expression (2), if the value of f₁/f₆ is less than the lower limit or greater than the upper limit, then the absolute value of distortion becomes too large, and chromatic aberration in magnification is harder to correct favorably.

The above-mentioned conditional expression (3) is one defining the ratio (v₁×v₃)^(½)/v₂ of the geometric mean of the Abbe number v₁ of the glass material of the first lens L₁ and the Abbe number v₃ of the glass material of the third lens L₃ to the Abbe number v₂ of the glass material of the second lens L₂, in order to favorably correct axial chromatic aberration.

In this conditional expression (3), if the value of (v₁×v₃) ^(½)/v₂ is less than the lower limit, then axial chromatic aberration is corrected insufficiently, whereby the focal positions of three colors of blue, green, and red shift in the axial direction beyond a practical range. If the value of (v₁×v₃) ^(½)/v₂ exceeds the upper limit, by contrast, axial chromatic aberration is corrected in excess, whereby the focal positions of three colors of blue, green, and red shift in the axial direction beyond a practical range.

The above-mentioned conditional expression (4) is one defining a range of a predetermined value of addition θ_(g,F)+0.0019v_(d) of the partial dispersion ratio θ_(g,F) of the glass material in each of the second lens L₂ and fifth lens L₅ and the Abbe number v_(d) of the glass material of each lens, in order to favorably correct axial chromatic aberration.

In this conditional expression (4), if the value of θ_(g,F)+0.0019v_(d) lies within the defined range, then axial chromatic aberration can be corrected to a high degree, so that the focal positions of three colors of blue, green, and red can be made very close to each other, whereby images can be read out with a high resolution.

The above-mentioned conditional expression (5) is one defining a range of a predetermined value of addition N_(d)+0.015v_(d) of the refractive index N_(d) at d-line and Abbe number v_(d) of the glass material in each of the first lens L₁ and sixth lens L₆, in order to favorably correct axial chromatic aberration.

In this conditional expression (5), if the value of N_(d)+0.015V_(d) lies within the defined range, axial chromatic aberration can be corrected to a high degree, so that the focal positions of three colors of blue, green, and red can be made very close to each other, whereby images can be read out with a high resolution.

The above-mentioned conditional expression (6) is one defining a relationship between the respective outside diameters φ₁₂, φ₃, φ₄, φ₅, and φ₆ of the individual lenses, such that the lenses can easily be inserted into a lens barrel, whereby axial misalignment is prevented from occurring.

In a conventional typical imaging lens, lenses located closer to the stop have a smaller outside diameter, so that, when the lenses are to be inserted into a lens barrel, those on the front side of the stop are inserted from the object side, whereas those on the rear side of the stop are inserted from the image side. In such a structure, both sides of the lens barrel from the stop are processed separately from each other, whereby it has been difficult to accurately align the respective optical axes of the lenses inserted from both sides of the lens barrel.

In the conventional image readout lens, even if such an axial misalignment is generated, its influence on optical performances remains within a permissible range of degree. When performances of a high resolution are required as in the image readout apparatus of the present invention, however, the axial misalignment is not negligible.

Therefore, if the above-mentioned conditional expression (6) is satisfied, then the lens barrel can be processed from one direction, whereas the lenses can be inserted from one direction. Though the stop is a separate component, causes for the above-mentioned misalignment can be eliminated, whereby lenses with a high resolution can be made stably.

The above-mentioned conditional expression (7) is one for defining the absolute value of magnification of the whole system.

Further, the lens in accordance with each example preferably satisfies the following conditional expression:

0.79<f ₄ /f<1.11  (8)

where

f₄ is the focal length of the fourth lens.

The above-mentioned conditional expression (8) is a condition for defining the ratio of the focal length of the fourth lens to that of the whole system, in order to maintain an optical symmetry between the front side and rear side from the stop.

In this conditional expression (8), if the value of f₄/f is less than the lower limit or greater than the upper limit, then the absolute value of distortion becomes greater, and chromatic aberration in magnification is harder to correct favorably.

Each of Examples 1 to 6 will now be explained with reference to specific values.

EXAMPLE 1

The upper part of the following Table 1 shows the radius of curvature R (mm) of each lens surface, center thickness of each lens and air space between each pair of neighboring lenses D (mm), refractive index N_(d) of each lens at d-line, Abbe number v_(d) of each lens, and glass material name of each lens in Example 1.

In Table 1 and Tables 2 to 6 which will be mentioned later, numbers referring to each of the letters R, D, N_(d), and v_(d) successively increase from the object side.

The middle part of Table 1 shows values of focal length f, F-number, half angle of view ω, and magnification β of the whole lens system in Example 1.

The lower part of Table 1 shows the respective values corresponding to the above-mentioned conditional expressions (1) to (4) and (8) in Example 1.

As can be seen from Table 1, Example 1 satisfies the above-mentioned conditional expressions (1) to (4), so that spherical aberration, curvature of field, astigmatism, distortion, chromatic aberration in magnification, and axial chromatic aberration are corrected sufficiently, whereby images can be read out with a high resolution.

Also, since the above-mentioned conditional expression (8) is satisfied, an optical symmetry between the front side and rear side of the lens from the stop can be maintained.

If the focal length of the whole lens system is set to about 80 mm in the image readout lens of Example 1, then an object extending over the shorter side of an A4-size sheet can be read out.

EXAMPLE 2

The upper part of the following Table 2 shows the radius of curvature R (mm) of each lens surface, center thickness of each lens and air space between each pair of neighboring lenses D (mm), refractive index N_(d) of each lens at d-line, Abbe number V_(d) of each lens, and glass material name of each lens in Example 2.

The middle part of Table 2 shows values of focal length f, F-number, half angle of view ω, and magnification β of the whole lens system in Example 2.

The lower part of Table 2 shows the respective values corresponding to the above-mentioned conditional expressions (1) to (4) and (8) in Example 2.

As can be seen from Table 2, Example 2 satisfies the above-mentioned conditional expressions (1) to (4), so that spherical aberration, curvature of field, astigmatism, distortion, chromatic aberration in magnification, and axial chromatic aberration are corrected sufficiently, whereby images can be read out with a high resolution.

Also, since the above-mentioned conditional expression (8) is satisfied, an optical symmetry between the front side and rear side of the lens from the stop can be maintained.

If the focal length of the whole lens system is set to about 80 mm in the image readout lens of Example 2, then an object extending over the shorter side of an A4-size sheet can be read out.

EXAMPLE 3

The upper part of the following Table 3 shows the radius of curvature R (mm) of each lens surface, center thickness of each lens and air space between each pair of neighboring lenses D (mm), refractive index N_(d) of each lens at d-line, Abbe number v_(d) of each lens, and glass material name of each lens in Example 3.

The middle part of Table 3 shows values of focal length f, F-number, half angle of view ω, and magnification β of the whole lens system in Example 3.

The lower part of Table 3 shows the respective values corresponding to the above-mentioned conditional expressions (1) to (3) and (8) in Example 3.

As can be seen from Table 3, Example 3 satisfies the above-mentioned conditional expressions (1) to (3), so that spherical aberration, curvature of field, astigmatism, distortion, chromatic aberration in magnification, and axial chromatic aberration are corrected sufficiently, whereby images can be read out with a high resolution.

Also, since the above-mentioned conditional expression (8) is satisfied, an optical symmetry between the front side and rear side of the lens from the stop can be maintained.

If the focal length of the whole lens system is set to about 80 mm in the image readout lens of Example 3, then an object extending over the shorter side of an A4-size sheet can be read out.

EXAMPLE 4

The upper part of the following Table 4 shows the radius of curvature R (mm) of each lens surface, center thickness of each lens and air space between each pair of neighboring lenses D (mm), refractive index N_(d) of each lens at d-line, Abbe number v_(d) of each lens, and glass material name of each lens in Example 4.

The middle part of Table 4 shows values of focal length f, F-number, half angle of view ω, and magnification β of the whole lens system in Example 4.

The lower part of Table 4 shows the respective values corresponding to the above-mentioned conditional expressions (1) to (3), (5), and (8) in Example 4.

As can be seen from Table 4, Example 4 satisfies the above-mentioned conditional expressions (1) to (3) and (5), so that spherical aberration, curvature of field, astigmatism, distortion, chromatic aberration in magnification, and axial chromatic aberration are corrected sufficiently, whereby images can be read out with a high resolution.

Also, since the above-mentioned conditional expression (8) is satisfied, an optical symmetry between the front side and rear side of the lens from the stop can be maintained.

If the focal length of the whole lens system is set to about 80 mm in the image readout lens of Example 4, then an object extending over the shorter side of an A4-size sheet can be read out.

Also, the glass material of each of the first lens L₁ to the sixth lens L₆ in the image readout lens of Example 4 includes no lead (Pb).

EXAMPLE 5

The upper part of the following Table 5 shows the radius of curvature R (mm) of each lens surface, center thickness of each lens and air space between each pair of neighboring lenses D (mm), refractive index N_(d) of each lens at d-line, Abbe number v_(d) of each lens, and glass material name of each lens in Example 5.

The middle part of Table 5 shows values of focal length f, F-number, half angle of view ω, and magnification β of the whole lens system in Example 5.

The lower part of Table 5 shows the respective values corresponding to the above-mentioned conditional expressions (1) to (3), (5), and (8) in Example 5.

As can be seen from Table 5, Example 5 satisfies the above-mentioned conditional expressions (1) to (3) and (5), so that spherical aberration, curvature of field, astigmatism, distortion, chromatic aberration in magnification, and axial chromatic aberration are corrected sufficiently, whereby images can be read out with a high resolution.

Also, since the above-mentioned conditional expression (8) is satisfied, an optical symmetry between the front side and rear side of the lens from the stop can be maintained.

If the focal length of the whole lens system is set to about 70 mm in the image readout lens of Example 5, then an object extending over the shorter side of an A3-size sheet can be read out.

Also, the glass material of each of the first lens L₁ to the sixth lens L₆ in the image readout lens of Example 5 includes no lead (Pb).

EXAMPLE 6

The upper part of the following Table 6 shows the radius of curvature R (mm) of each lens surface, center thickness of each lens and air space between each pair of neighboring lenses D (mm), refractive index N_(d) of each lens at d-line, Abbe number v_(d) of each lens, and glass material name of each lens in Example 6.

The middle part of Table 6 shows values of focal length f, F-number, half angle of view ω, and magnification β of the whole lens system in Example 6.

The lower part of Table 6 shows the respective values corresponding to the above-mentioned conditional expressions (1) to (3) and (8) in Example 6.

As can be seen from Table 6, Example 6 satisfies the above-mentioned conditional expressions (1) to (3), so that spherical aberration, curvature of field, astigmatism, distortion, chromatic aberration in magnification, and axial chromatic aberration are corrected sufficiently, whereby images can be read out with a high resolution.

Also, since the above-mentioned conditional expression (8) is satisfied, an optical symmetry between the front side and rear side of the lens from the stop can be maintained.

If the focal length of the whole lens system is set to about 70 mm in the image readout lens of Example 6, then an object extending over the shorter side of an A3-size sheet can be read out.

Also, the glass material of each of the first lens L₁ to the sixth lens L₆ in the image readout lens of Example 6 includes no lead (Pb).

FIGS. 2 to 13 show aberration charts (for spherical aberration, astigmatism, distortion, chromatic aberration in magnification, and lateral aberration) corresponding to Examples 1 to 6. Here, each chart of spherical aberration shows aberrations with respect to e-line, g-line, and C-line, whereas each chart of chromatic aberration in magnification shows aberrations with respect to g-line and C-line. Also, each aberration chart of astigmatism shows aberrations with respect to sagittal (S) image surface and tangential (T) image surface at each wavelength.

Here, each aberration chart of Examples 1 to 4 is obtained in the state where a glass plate having a thickness of 2.4 mm is contained between the object surface and the lens, whereas a glass plate having a thickness of 0.64 mm is contained between the lens and the image surface. Also, each aberration chart of Examples 5 and 6 is obtained in the state where a glass plate having a thickness of 2.1 mm is contained between the object surface and the lens, whereas a glass plate having a thickness of 0.56 mm is contained between the lens and the image surface.

As can be seen from FIGS. 2 to 13, all the above-mentioned aberrations can be made favorable in accordance with each of the above-mentioned Examples.

Though the first lens L₁ and the second lens L₂ are cemented to each other in each of the above-mentioned Examples, similar performances can also be obtained when the first lens L₁ and the second lens L₂ are independent from each other without being cemented.

Though specific values are mentioned in each of the above-mentioned Examples, the absolute value of the magnification of the whole lens system, |β|, may be a value other than those mentioned above, and can appropriately be set within the range of 0.05≦|β|≦0.7. For example, any value of |β|=0.6, |β|=0.5, |β|=0.1, |β|=0.05, and the like may be employed. Among these values, a range of 0.09≦|β|≦0.4 can be mentioned as one often employed in such a kind of imaging lens in general.

Without being restricted to those of the above-mentioned embodiment, the image readout lens of the present invention can be modified in various manners. For example, the radius of curvature R and lens space (or lens thickness) D of each lens can be changed as appropriate.

While various image readout apparatus, such as facsimile machine and color scanner, mounted with the image readout lens of this embodiment are used for reading out images from the original, the quality of the read-out images becomes favorable at this time.

As explained in the foregoing, since the imaging lens of the present invention has a five-group, six-element configuration as a whole and satisfies the above-mentioned predetermined conditional expressions, it can favorably correct various aberrations, and can lower the curvature of field in particular.

Therefore, as the depth is made greater, the burden of focus adjustment in the image readout apparatus can be alleviated, and the deterioration in resolution due to fluctuations in flatness of the original or due to the flotation thereof decreases.

Also, as the relationship between the respective outside diameters of the individual lenses is defined, the axial misalignment of the lenses can be eliminated, whereby a lens with a high resolution can be manufactured stably.

Further, the image readout apparatus using the image readout lens of this embodiment can yield a favorable image quality at the time of image readout.

TABLE 1 Surface R D Nd νd Glass material name 1 20.041 7.85 1.62041 60.3 SK-16(SUMITA) 2 150.405 0.0 3 150.405 2.0 1.6134 44.3 BPM51(OHARA) 4 18.996 1.875 5 47.703 3.749 1.63854 55.5 SK-18(SUMITA) 6 114.175 9.112 7 −35.279 4.077 1.63854 55.5 SK-18(SUMITA) 8 −22.741 1.872 9 −15.092 1.999 1.6134 44.3 BPM51(OHARA) 10 −98.416 0.694 11 −104.915 7.43 1.62041 60.3 SK-16(SUMITA) 12 −21.281 f = 100 mm, F/6.3, ω = 20.9°, β = −0.37795 (1) f₃₄/f = 0.572 (2) f₁/f₆ = 0.977 (3) (ν₁ × ν₃) ^(½)/ν₂ = 1.306 (4) θ_(g,F) + 0.0019 ν_(d) = 0.645 (8) f₄/f = 0.889 where, in the second lens L₂ and in the fifth lens L₅, N_(g) = 1.63088,N_(F) = 1.62310,N_(c) = 1.60924

TABLE 2 Surface R D N_(d) ν_(d) Glass material name 1 22.204 8.176 1.62041 60.3 SK-16(SUMITA) 2 168.882 0.0 3 168.882 1.998 1.6134 44.3 BPM51(OHARA) 4 19.144 1.873 5 47.948 3.746 1.63854 55.5 SK-18(SUMITA) 6 113.274 9.082 7 −35.565 4.168 1.63854 55.5 SK-18(SUMITA) 8 −22.856 1.873 9 −15.016 1.998 1.6134 44.3 BPM51(OHARA) 10 −96.327 1.068 11 −103.634 6.847 1.62041 60.3 SK-16(SUMITA) 12 −21.142 f = 100 mm, F/6.3, ω = 20.8°, β = −0.37795 (1) f₃₄/f = 0.574 (2) f₁/f₆ = 0.972 (3) (ν₁ × ν₃) ^(½)/ν₂ = 1.306 (4) θ_(g,F) + 0.0019 ν_(d) = 0.645 (8) f₄/f = 0.888 where, in the second lens L₂ and in the fifth lens L₅ N_(g) = 1.63088,N_(F) = 1.62310,N_(c) = 1.60924

TABLE 3 Surface R D N_(d) ν_(d) Glass material name 1 24.277 8.132 1.65844 50.8 SSK-5(SUMITA) 2 112.017 0.0 3 112.017 3.1156 1.63042 38.0 F-5(SUMITA) 4 20.454 1.869 5 59.778 3.738 1.58913 61.2 SK-5(SUMITA) 6 168.006 9.208 7 −34.744 4.299 1.58913 61.2 SK-5(SUMITA) 8 −22.184 1.877 9 −15.596 1.994 1.60342 38.0 F-5(SUMITA) 10 −65.37 2.492 11 −75.55 6.599 1.65844 50.8 SSK-5(SUMITA) 12 −23.432 f = 100 mm, F/6.3, ω = 20.8°, β = −0.37795 (1) f₃₄/f = 0.629 (2) f₁/f₆ = 0.924 (3) (ν₁ × ν₃) ^(½)/ν₂ = 1.467 (8) f₄/f = 0.924

TABLE 4 Surface R D N_(d) ν_(d) Glass material name 1 23.607 8.712 1.5924 68.3 GFK-68(SUMITA) 2 75.478 0.0 3 75.478 3.112 1.54814 45.8 S-TIL1(OHARA) 4 19.839 1.867 5 58.681 3.734 1.62041 60.3 SK-16(SUMITA) 6 160.115 9.133 7 −33.645 4.294 1.62041 60.3 SK-16(SUMITA) 8 −22.065 1.92 9 −15.154 1.991 1.54814 45.8 S-TIL1(OHARA) 10 −68.358 2.489 11 −73.909 7.059 1.5924 68.3 GFK-68(SUMITA) 12 −22.345 f = 100 mm, F/6.3, ω = 20.8°, β = −0.37795 (1) f₃₄/f = 0.610 (2) f₁/f₆ = 1.061 (3) (ν₁ × ν₃) ^(½)/ν₂ = 1.398 (5) N_(d) + 0.015 ν_(d) = 2.617 (8) f₄/f = 0.905

TABLE 5 Surface R D N_(d) ν_(d) Glass material name 1 23.515 8.771 1.5429 68.3 GFK-68(SUMITA) 2 84.233 0.0 3 84.233 3.132 1.54814 45.8 S-TIL1(OHARA) 4 20.095 2.148 5 55.288 3.759 1.62041 60.3 SK-16(SUMITA) 6 142.34 8.771 7 −32.324 5.179 1.62041 60.3 SK-16(SUMITA) 8 −22.106 2.147 9 −15.329 2.005 1.54814 45.8 S-TIL1(OHARA) 10 −67.779 2.374 11 −77.719 7.16 1.5924 68.3 GFK-68(SUMITA) 12 −23.011 f = 100 mm, F/5.0, ω = 18.9°, β = −0.18898 (1) f₃₄/f = 0.624 (2) f₁/f₆ = 0.993 (3) (ν₁ × ν₃) ^(½)/ν₂ = 1.398 (5) N_(d) + 0.015 ν_(d) = 2.617 (8) f₄/f = 0.944

TABLE 6 Surface R D N_(d) ν_(d) Glass material name 1 28.596 8.8 1.713 53.9 S-LAL8(OHARA) 2 66.422 0.0 3 66.422 3.143 1.59551 39.2 S-TIM8(OHARA) 4 23.194 2.515 5 56.429 3.772 1.713 53.9 S-LAL8(OHARA) 6 118.646 10.847 7 −37.543 4.337 1.713 53.9 S-LAL8(OHARA) 8 −25.826 1.886 9 −18.592 3.143 1.59551 39.2 S-TIM8(OHARA) 10 −58.019 0.377 11 −66.192 8.122 1.713 53.9 S-LAL8(OHARA) 12 −27.86 f = 100 mm, F/5.0, ω = 18.9°, β = −0.18898 (1) f₃₄/f = 0.656 (2) f₁/f₆ = 1.036 (3) (ν₁ × ν₃) ^(½)/ν₂ = 1.375 (8) f₄/f = 1.006 

What is claimed is:
 1. An imaging lens comprising, successively from an object side, a positive first lens having a convex surface directed onto the object side, a negative second lens having a concave surface directed onto an image side, a third lens made of a positive meniscus lens having a convex surface directed onto the object side, a fourth lens made of a positive meniscus lens having a convex surface directed onto the image side, a negative fifth lens having a concave surface directed onto the object side, and a positive sixth lens having a convex surface directed onto the image side, said first and second lenses being cemented to each other, said second and fifth lenses satisfying the following conditional expression (4): θ_(g,F)+0.0019v _(d)<0.650  (4) where θ_(g,F) is a partial dispersion ratio of a lens material expressed by: θ_(g,F)=(N _(g) −N _(F))/(N _(F) −N _(C)) v_(d) is an Abbe number of the lens material expressed by: v _(d)=(N _(d)−1)/(N _(F) −N _(C)) where N_(g) is the refractive index of the lens material at a wavelength of 435.8 nm; N_(F) is the refractive index of the lens material at a wavelength of 486.1 nm; N_(C) is the refractive index of the lens material at a wavelength of 656.3 nm; and N_(d) is the refractive index of the lens material at a wavelength of 587.6 nm.
 2. An imaging lens according to claim 1, wherein said imaging lens satisfies the following conditional expression (6): φ₆≦φ₅≦φ₄≦φ₃≦φ₁₂ or φ₁₂≦φ₃≦φ₄≦φ₅≦φ₆  (6) where φ₁₂ is the larger outside diameter in any of the first and second lenses; φ₃ is the outside diameter of the third lens; φ₄ is the outside diameter of the fourth lens; φ₅ is the outside diameter of the fifth lens; and φ₆ is the outside diameter of the sixth lens.
 3. An imaging lens according to claim 1, wherein said imaging lens satisfies the following conditional expression (7): 0.05≦|β|≦0.7  (7) where β is the magnification of the whole system.
 4. An image readout apparatus using the imaging lens according to claim
 1. 5. An imaging lens comprising, successively from an object side, a positive first lens having a convex surface directed onto the object side, a negative second lens having a concave surface directed onto an image side, a third lens made of a positive meniscus lens having a convex surface directed onto the object side, a fourth lens made of a positive meniscus lens having a convex surface directed onto the image side, a negative fifth lens having a concave surface directed onto the object side, and a positive sixth lens having a convex surface directed onto the image side, said first and second lenses being cemented to each other, said first and sixth lenses satisfying the following conditional expression (5): N _(d)+0.015v _(d)>2.58  (5) where N_(d) is the refractive index of a lens material at d-line; and v_(d) is the Abbe number of the lens material.
 6. An imaging lens according to claim 5, wherein said imaging lens satisfies the following conditional expression (6): φ₆≦φ₅≦φ₄≦φ₃≦φ₁₂ or φ₁₂≦φ₃≦φ₄≦φ₅≦φ₆  (6) where φ₁₂ is the larger outside diameter in any of the first and second lenses; φ₃ is the outside diameter of the third lens; φ₄ is the outside diameter of the fourth lens; φ₅ is the outside diameter of the fifth lens; and φ₆ is the outside diameter of the sixth lens.
 7. An imaging lens according to claim 5, wherein said imaging lens satisfies the following conditional expression (7): 0.05≦|β|≦0.7  (7) where β is the magnification of the whole system.
 8. An image readout apparatus using the imaging lens according to claim
 5. 9. An imaging lens comprising, successively from an object side, a positive first lens having a convex surface directed onto the object side, a negative second lens having a concave surface directed onto an image side, a third lens made of a positive meniscus lens having a convex surface directed onto the object side, a fourth lens made of a positive meniscus lens having a convex surface directed onto the image side, a negative fifth lens having a concave surface directed onto the object side, and a positive sixth lens having a convex surface directed onto the image side, said first and second lenses being cemented to each other, said imaging lens satisfying the following conditional expressions (1) to (3): 0.51<f ₃₄ /f<0.72  (1) 0.83<f ₁ /f ₆<1.17  (2) 1.17<(v ₁ ×v ₃)^(½) /v ₂<1.62  (3) where f is the focal length of the whole system; f₃₄ is the composite focal length of the third and fourth lenses; f₁ is the focal length of the first lens; f₆ is the focal length of the sixth lens; v₁ is the Abbe number of a material of the first lens; v₂ is the Abbe number of a material of the second lens; and v₃ is the Abbe number of a material of the third lens, wherein said imaging lens satisfies at least one of the following conditional expression (4) concerning said second and fifth lenses and the following conditional expression (5) concerning said first and sixth lenses: θ_(g,F)+0.0019v _(d)<0.650  (4) where θ_(g,F) is a partial dispersion ratio of a lens material expressed by: θ_(g,F)=(N _(g) −N _(F))/(N _(F) −N _(C)) v_(d) is an Abbe number of the lens material expressed by: v _(d)=(N _(d)−1)/(N _(F) −N _(C)) where N_(g) is the refractive index of the lens material at a wavelength of 435.8 nm, N_(F) is the refractive index of the lens material at a wavelength of 486.1 nm, N_(C) is the refractive index of the lens material at a wavelength of 656.3 nm, and N_(d) is the refractive index of the lens material at a wavelength of 587.6 nm; N _(d)+0.015v _(d)<2.58  (5) where N_(d) is the refractive index of a lens material at a d-line, and v_(d) is the Abbe number of the lens material.
 10. An image readout apparatus using the imaging lens according to claim
 9. 11. An imaging lens comprising, successively from an object side, a positive first lens having a convex surface directed onto the object side, a negative second lens having a concave surface directed onto an image side, a third lens made of a positive meniscus lens having a convex surface directed onto the object side, a fourth lens made of a positive meniscus lens having a convex surface directed onto the image side, a negative fifth lens having a concave surface directed onto the object side, and a positive sixth lens having a convex surface directed onto the image side, said first and second lenses being cemented to each other, said imaging lens satisfying the following conditional expressions (1) to (3): 0.51<f ₃₄ /f<0.72  (1) 0.83<f ₁ /f ₆<1.17  (2) 1.17<(v ₁ ×v ₃)^(½) /v ₂<1.62  (3) where f is the focal length of the whole system; f₃₄ is the composite focal length of the third and fourth lenses; f₁ is the focal length of the first lens; f₆ is the focal length of the sixth lens; v₁ is the Abbe number of a material of the first lens; v₂ is the Abbe number of a material of the second lens; and v₃ is the Abbe number of a material of the third lens, wherein said imaging lens satisfies the following conditional expression (6): φ₆≦φ₅≦φ₄≦φ₃≦φ₁₂ or φ₁₂≦φ₃≦φ₄≦φ₅≦φ₆  (6) where φ₁₂ is the larger outside diameter in any of the first and second lenses; φ₃ is the outside diameter of the third lens; φ₄ is the outside diameter of the fourth lens; φ₅ is the outside diameter of the fifth lens; and φ₆ is the outside diameter of the sixth lens.
 12. An image readout apparatus using the imaging lens according to claim
 11. 13. An imaging lens comprising, successively from an object side, a positive first lens having a convex surface directed onto the object side, a negative second lens having a concave surface directed onto an image side, a third lens made of a positive meniscus lens having a convex surface directed onto the object side, a fourth lens made of a positive meniscus lens having a convex surface directed onto the image side, a negative fifth lens having a concave surface directed onto the object side, and a positive sixth lens having a convex surface directed onto the image side, said first and second lenses being cemented to each other, said imaging lens satisfying the following conditional expressions (1) to (3): 0.51<f ₃₄ /f<0.72  (1) 0.83<f ₁ /f ₆<1.17  (2) 1.17<(v ₁ ×v ₃)^(½) /v ₂<1.62  (3) where f is the focal length of the whole system; f₃₄ is the composite focal length of the third and fourth lenses; f₁ is the focal length of the first lens; f₆ is a the focal length of the sixth lens; v₁ is the Abbe number of a material of the first lens; v₂ is the Abbe number of a material of the second lens; and v₃ is the Abbe number of a material of the third lens, wherein said imaging lens satisfies the following conditional expression (7): 0.05≦|β|≦0.7  (7) where β is the magnification of the whole system.
 14. An image readout apparatus using the imaging lens according to claim
 13. 